(FMT:4908): GStreamer-CRITICAL **: gst_element_set_state: assertion 'GST_IS_ELEMENT (element)' failed
my host is ubuntu 16.04, and my Qt is 5.7.1 version.
i try to play a .wmv with my project. and i can play the file with double-click
can someone helps me?

@jsulm
thank you, i just install all the plugins and there is no such error
. I try to play the media .wmv with GStreamer command line client.
it works well . but anther erro happen when i try to play the medias, error info:
Cannot connect to server socket err = No such file or directory
Cannot connect to server request channel
jack server is not running or cannot be started
JackShmReadWritePtr::~JackShmReadWritePtr - Init not done for 4294967295, skipping unlock
JackShmReadWritePtr::~JackShmReadWritePtr - Init not done for 4294967295, skipping unlock
Cannot connect to server socket err = No such file or directory
Cannot connect to server request channel
jack server is not running or cannot be started

so i use sudo apt-cache search gstreamer* , and install many plugins,
including gstreamer1.0-plugins-ugly, gstreamer1.0-plugins-bad, gstreamer1.0-plugin-good
now there is only one warning:
GStreamer: unable to pause pause- "file:///home/tong/src/Wildlife.wmv"
warning: no decoder available for type "video/x-ms-asf"
seems need to install decoders...

another question:
As my target is rapsberry pi3,i try to play video with default GStreamer command，the video is not fluency. i need to enable hardware-accelerate when using QT to play video, Does QT support this feature when play video? how to enable it ? thank you !

@jsulm@SGaist@mrjj
there seems something strange, as Gstreamer0.10-plugins-ugly is no longer available, when i remove all gstreamer0.1 packages, i try to run my project, there is info:
defaultserviceprovider::requestservice(): no service found for - "org.qt-project.qt.mediaplayer".
then i reinstall all gstreamer0.1 packages.
the info is :
GStreamer: unable to pause pause- "file:///home/tong/src/Wildlife.wmv"
warning: no decoder available for type "video/x-ms-asf"
does it mean QT still use gstreamer0.1 package?
but gstreamer0.10-plugins-bad, gstreamer0.10-plugins-ugly no long available.
i have installed gstreamer1.0-plugins-ugly, gstreamer1.0-plugins-bad, gstreamer1.0-plugin-good.

@jsulm
i have checked the four repositories in system settings ->software& updates->ubuntu software tab.
but failed to run sudo apt-get install gstreamer0.10-plugins-ugly
info:
Package gstreamer0.10-plugins-ugly is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

@SGaist
well ,Thank you
i will try to compile QT5.7.1 against GStreamer 1.0. can you give me some instructions? thank you!
another thing is how to support hardware-accelerate? or it support it by default,but the default behavior of Gstreamer seem not enable hardware-accelerate.
my project finally runs on raspberry pi3, i do a lot efforts to make the QT5.7 deployed on rpi3. i'm worried about QT5.8....
i find a description:
1,install gstream1.0*
install libgstreamer1.0*
2, cd qt5.7.1
run ./configure -gstreamer 1.0
make
make install
is it right?

@SGaist@jsulm@mrjj
i finish the reinstall work with all modules rebuild,the output result is still NG.
play the video with gstreamer command the output is below:
gst-play-1.0 /home/tong/build-FMT-gstream1_0-Debug/src/Wildlife.wmv
Press 'k' to see a list of keyboard shortcuts.
Now playing /home/tong/build-FMT-gstream1_0-Debug/src/Wildlife.wmv
libva info: VA-API version 0.39.0
libva info: va_getDriverName() returns 0
libva info: Trying to open /usr/lib/x86_64-linux-gnu/dri/i965_drv_video.so
libva info: Found init function __vaDriverInit_0_39
libva info: va_openDriver() returns 0
Redistribute latency...
Redistribute latency...
Redistribute latency...
0:00:30.0 / 0:00:30.0
Reached end of play list.the video can play normally.

@SGaist@jsulm
currently , i just test on my ubuntu. my host is x64bit. i install gstreamer1.0 with command sudo apt-get install gstreamer1.0
and can help give steps about how to rebuild only qtmultimedia module with gstreamer1.0?

@SGaist
the version is QT5.7.1 i have install with QtMultimedia using gstreamer0.1.
i try to rebuild the QtMultimedia module with the version i already installed,
should i parse what parameters to let the QtMultimedia module know that it should use gstreamer1.0 instead of gstreamer0.1? can you give instructions please?

@SGaist@jsulm
thank you for your great support,
but i found a strange thing:
i can not play the .wmv or .mp4 video even when i try to use QT5.8(install by qt-opensource-linux-x64-5.8.0.run, host is ubuntu 16.04),
after i install Qt5.8, i use the Qtcreator4.2.1 to open the project player under multimediawidgets Example, run the example, and open a Wildlife.wmv(example video under windows 7), try to play it ,just no video .output is :
libva info: VA-API version 0.39.0
libva info: va_getDriverName() returns 0
libva info: Trying to open /usr/lib/x86_64-linux-gnu/dri/i965_drv_video.so
libva info: Found init function __vaDriverInit_0_39
libva info: va_openDriver() returns 0

I installed gstreamer with following commands:
1,sudo apt-get install gstreamer1.0
2,sudo apt-get install libgstreamer-plugins-good1.0-dev libgstreamer-plugins-base1.0-dev libgstreamer1.0-dev libgstreamer1.0-0 libqgsttools-p1 libqt5multimedia5-plugins gstreamer1.0-libav libges-1.0-dev libgstreamer-plugins-bad1.0-dev libgstreamermm-1.0-0v5 libgstreamermm-1.0-dev libqt5gstreamer-1.0-0 libqt5gstreamer-dev libqt5gstreamerquick-1.0-0 libqt5gstreamerui-1.0-0 libqt5gstreamerutils-1.0-0 libqtglib-2.0-0 libqtgstreamer-1.0-0 libqtgstreamer-dev libqtgstreamerui-1.0-0 libqtgstreamerutils-1.0-0 qtgstreamer-plugins-qt5 qtgstreamer-plugins ubuntu-restricted-extras
can you check it for me? the video is OK, i can play it with gst-play-1.0 or with double click.
ps: i try to play a *.mp3, it works well, and there are sounds playing with no pictures if the format is mp4